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Le Touquet – Paris-Plage, France

Berest P.,Ecole Polytechnique - Palaiseau | Brouard B.,Brouard Consulting
48th US Rock Mechanics / Geomechanics Symposium 2014

Four in-situ tests performed in salt caverns in France and Germany are described. The main objective of these tests was to increase our understanding of the long-term behavior of abandoned caverns. It is proven that, in the long term, when cavern brine has reached thermal equilibrium with the rock mass, pressure evolution is governed by cavern creep closure and brine micro- permeation through the cavern walls. An equilibrium pressure is reached when the closure rate exactly equals the permeation rate. In the shallow caverns described in this paper, equilibrium pressure is significantly lower than geostatic pressure, ruling out any risk of fracture onset at the cavern roof. Interpretation of these tests allows salt permeability to be back-calculated. Copyright © 2014 ARMA, American Rock Mechanics Association. Source

Berest P.,Ecole Polytechnique - Palaiseau | Brouard B.,Brouard Consulting
Mechanical Behavior of Salt VIII - Proceedings of the Conference on Mechanical Behavior of Salt, SALTMECH VIII

In some laboratory tests, it was observed that after a rapid stress drop, the sign of the strain rate changed. A simple constitutive model is proposed that accounts for this phenomenon, which is called “rheological reverse creep”. Reverse creep also is observed following a rapid pressure increase in a salt cavern: in this environment, cavern volume increases over several days even when pressure is kept constant after the pressure increase. This phenomenon, however, results from different factors, among which rheological reverse creep is only one: numerical computations prove that the slow redistribution of stresses in the rock mass, or “geometrical reverse creep”, also plays a significant role. © 2015 Taylor & Francis Group, London. Source

Berest P.,Ecole Polytechnique - Palaiseau | Brouard B.,Brouard Consulting | Hevin G.,Storengy
EPJ Web of Conferences

In 1997-1998, an abandonment test was performed in a 950-m deep, 8000-m3 salt cavern operated by GDF SUEZ at Etrez, France. In this relatively small brine-filled cavern, which had been kept idle for 15 years before the test, thermal equilibrium was reached. A special system was designed to monitor leaks, which proved to be exceedingly small. In these conditions, brine permeation and cavern creep closure are the only factors to play significant roles in pressure evolution. This test strongly suggested that obtaining an equilibrium pressure such that the effects of these two factors were exactly equal would be reached in the long term. Four years later, pressure monitoring in the closed cavern resumed. Pressure evolution during the 2002-2009 period confirmed that cavern brine pressure will remain constant and significantly smaller than geostatic pressure in the long term, precluding any risk of fracturing and brine seepage to the overburden layers. © 2010 Owned by the authors, published by EDP Sciences. Source

Thoraval A.,INERIS | Lahaie F.,INERIS | Brouard B.,Brouard Consulting | Berest P.,Ecole Polytechnique - Palaiseau
International Journal of Rock Mechanics and Mining Sciences

The most accepted strategy for abandoning solution-mined salt storage caverns involves filling the cavern with brine and sealing the well permanently. The sealing can be delayed waiting for the brine to reach thermal equilibrium with the surrounding salt. This concept is based on the principle that the cavern, once closed, will reach an equilibrium pressure that will assure the cavern's long-term mechanical stability.This article provides quantitative information about the evolution of abandoned salt caverns (the value of the equilibrium pressure, the time before thermal equilibrium is achieved, the rate of brine expulsion from the cavern, the time before the cavern closes up) from numerical simulations based on the state of the art related to the phenomena that affect the cavern after it is abandoned. The model reveals the conditions under which an excessive increase in the brine pressure in the cavern may appear, which can lead to the walls of the cavern being damaged. It also provides a basis for recommendations that could help to control these risks. © 2015 Elsevier Ltd. Source

Berest P.,Ecole Polytechnique - Palaiseau | Beraud J.F.,Ecole Polytechnique - Palaiseau | Gharbi H.,Ecole Polytechnique - Palaiseau | Brouard B.,Brouard Consulting | DeVries K.,RESPEC
Rock Mechanics and Rock Engineering

The applied deviatoric stress during most creep tests performed on salt samples is in the 3.5–20 MPa range. However, the stresses actually experienced in the vicinity of a salt cavern are much smaller. Any extrapolation is difficult to vindicate, as the dominant micro-mechanisms are strongly suspected to be very different in the low-stress and medium-stress domains. To answer this concern, a very slow creep test was performed on an Avery Island salt sample. To minimize the influence of even the smallest of temperature deviations during the test, the testing apparatus was placed in a remote gallery of the Varangéville salt mine, taking advantage of the very stable temperature conditions offered in an underground environment. The test was performed in multiple stages and lasted 42 months. The successive loads of 0.1, 0.2, 0.3, and 0.5 MPa were applied. Measured steady-state strain rates were of the order of 10−12 s−1, which are significantly faster than that extrapolated from creep tests performed at loads ranging between 3.5 and 20 MPa. © 2015, Springer-Verlag Wien. Source

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